Radar processor having adaptive clutter rejection

ABSTRACT

A radar signal processing system wherein clutter signals which are continually varying in amplitude and spectral distribution are cancelled in an optimum manner by means adaptive to particular clutter conditions, thereby to permit effective target detection and low false alarm rate operation in the presence of such clutter. An electronically variable high pass filter is employed, the response of which automatically adjusts to input clutter conditions to provide the required amount of clutter cancellation while at the same time affording optimal target detection.

United States Patent [72] Inventor Aaron A. Galvin 3,776,426 1/1957Altman 343/7.7X Lexington, Mass. 2,870,331 l/l959 Gray 343/17.1X [21]Appl. No. 783,289 3,312,969 4/1967 Halstead 343/17.1X [22] Filed Dec.12,1968 3,149,333 9/1964 Campbell 343/l7.1 [45] Patented Apr. 27, 1971OTHER REFERENCES [73] Asslgnee Aerospace Research Skolnik, Introductionto Radar Systems, 1962 pp. 117- 18.

Boston, Mass.

l Primary Examiner- Rodney D. Bennett, Jr. [54] RADAR PROCESSOR HAVINGADAPTIVE Assistant Examiner-Joseph G. Baxter CLU'I'IER REJECTIONAttorney-Joseph Weingarten 12 Claims, 6 Drawing Figs.

[52] Us. Cl 343/7], ABSTRACT: A radar Signal processing System whereinclutter 343/17-1 signals which are continually varying in amplitude andspec- [51] f Cl G015 9/42 tral distribution are cancelled in an optimummanner by [50] Field ofSearch 343/7], means d ti t ti l l tt diti th b tpermit effective target detection and low false alarm rate operation inthe presence of such clutter. An electronically [56] References cuedvariable high pass filter is employed, the response of which au- UNITEDSTATES PATENTS tomatically adjusts to input clutter conditions toprovide the 2,871,468 1 1959 Smith 343/7.7X required amount of cluttercancellation while at the same time 3,267,468 8/1966 Stull 343/7.7affording optimal target detection.

LOW N0 ELECT'FIONKJALLY AUDO PASS VARIABLE 1 2s SIGNALS FILTER HIGH PASSFROM RADAR FILTER AMPLIFIER l r' l 28 AMPLIFIER DETECTOR T|MEONTARGETDETECTOR MATCHED INTEGRATOR CLUTJEER 4 32 RES! U INTEGRATOR TR'GGER I4 124 l CONTROL ALARM RESET CKT PATENTED APR27|97| 3; 576, 564

SHEET 1 BF 3 LOW WIND SPECTRAL MEDIUM WIND DENSITY HIGH wIN F I G IMOVING TARGETS DOPPLER FREQUENCY [\K) l2 Low ELECTRONICALLY PASSVARIABLE V 26 FILTER FROM RA AR 'EE S AMPLIFIER l I AMPLIFIER DETEcToRTIMEONTARGET F I G 2 DETECTOR MATCHED I V 22 INTEGRATOR CLUTTER 32REsIDuE A INTEGRATOR TRIGGER 24 CONTROL ALARM REsET CKT K FROM 38coNTRDL CONTROLLER CKT g I I I r36 FROM ApTIvE FILT IR I F'LTER STAGE lSTAGE 2 I STAGE 3 INvENToR ARON A. GALVTN TO CONTROL TO TARGET B LOOP l4DETECTION F l6. 3 CHANNEL 16 M TORN YS PATENTEn IPrIz 1 IQII SHEET 2 BF3 FROM ELECTRoNICALLY VARIABLE FILTER l2 I 400 4Ob {/406 Low' PAss BANDPASS HIGH PASS FILTER FILTER FILTER I I f D TECTOR DETECTOR DETECTOR I(/440 V /44!) l I 44c 'MATCHED MATCHED MATCHED INTEGRATOR INTEGRATORINTEGRAToR OR CIRCUIT F G. 4 TRIGGER \/48 ALARM so L0G FREQUENCY 2 \Y\ 85a 62 ff, o: 8 ..1

MIN. CUTOFF BAND PASS Low PASS HIGH PASS Low PASS FILTER FILTER FILTERFILTER FILTER I2 40b 40c 40c Io MAX. CuToFF INVENTOR FIG. 5 AARON A.GALVIN ATTORNEYS PATENTED mm I971 SHEET 3 OF 3 QUADRATURE AUDIO SIGNALSFROJM RADAR 4 ELECTRONICALLY ELECTRONICALLY VARIABLE VARIABLE J E WFILTER FILTER cLuTTER CLUTTER CONTROL CONTROL LOOP. LOOP SINGLE-SIDEBANDAUDIO \72 CONVERTER E 7 74 FILTER BANK FILTER BANK 6 V DETECTORSDETECTORS 8O SUMMING |NTEGRATORS\/84 o R 86 CIRCUIT TRIGGER /aa F IG. 6

INVENTOR ALARM AARON A. GALVIN ATTO NEYS I RADAR PROCESSOR HAVINGADAPTIVE CLU'I'IER REJECTION FIELD OF THE INVENTION This inventionrelates to radar systems and more particularly to Doppler radar systemshaving means for detecting moving targets in the presence of clutter.

BACKGROUND OF THE INVENTION In a Doppler radar system it is important todiscriminate signals attributable to a target from signals attributableto clutter. Any radar system which is attempting to detect or track atarget which is moving along the ground is subject to radar backscatterenergy from the local terrain as well as from the target of interest.This terrain backscatter, which may be reflections from natural objectssuch as trees, foliage, rocks, hills, etc. or manmade objects such asbuildings represents a source'of radarsignals which tend to mask thetarget of interest, these spurious signals being referred to as clutter.In a marine environment, clutter is caused by backscatter from the sea.The level of clutter can vary substantially depending on many factorssuch as the size of the radar resolution cell, the roughness of theterrain or sea, the amount of foliage and the angular disposition of theradar beam relative to the terrain. In addition to variation of clutterlevel, the distribution of clutter power in Doppler frequency, that isthe clutter spectrum, can vary considerably. If a radar views the groundwith a stationary antenna and there is no wind present, the clutterspectrum will be narrowly constrained in Doppler frequency, the width ofthis spectrum being determined only by small instabilities in the radarsystem. However, there are a number of factors which can cause thisspectrum to broaden considerably, such as wind which can cause motion inthe foliage or, at sea, cause increased wave activity, rotation of theradar antenna at a fixed location or translation of the radar antennasuch as might occur when the antenna is supported on an airborne orshipborne platform.

The large variation in clutter spectral characteristics causesdifficulties in Doppler radar systems in that the amount of clutter thatis passed by the radars signal processing equipment can be highlyvariable and under some circumstances can result in a high false alarmrate, that is, the false detection of clutter as true targets.

In general, clutter reduction in conventional systems has been attemptedby signal processing techniques based on assumptions concerning theexpected spectrum and amplitude level of clutter. Moreover, asdescribed, the clutter spectrum and level varies considerably inpractice depending upon the cause and particular nature of the clutter,and therefore a priori knowledge of clutter spectra or level is notavailable for optimum clutter rejection.

The false alarm rate has been controlled to some extent in conventionalradar systems by use of fixed Doppler filters which are designed to passonly high Doppler frequencies, or

by varying the gain of a signal processing channel in response tovariations in the level of clutter passing through the radar filters.However, the use of fixed filters sacrifices the detection of lowvelocity targets for a lower false alarm rate, while the use of gainvariation reduces the overall sensitivity of the system to targets inthe presence of clutter of wide spectral spread. Thus, in a dynamicenvironment in which the clutter spectral spread varies greatly,conventional fixed filter systems often lack effective clutter rejectionand therefore suffer high false alarm rates.

SUMMARY OF THE INVENTION In accordance with the present invention, aradar processor is provided which utilizes a measure of clutter spectrumpresent in a particular interval of time to set the cutoff frequency ofan electronically variable filter which dynamically maintains anacceptable clutter output level as its spectrum varies with operatingconditions. Briefly, a radar processor according to the inventionincludes an electronically variable high pass filter having a firstoutput coupled to a feedback control channel and a second output coupledto a target signal detection channel. The control channel is operativeto derive a control signal from the Doppler input signals which is fedback to the variable filter to adjust its cutoff frequency in accordancewith input signal conditions such that clutter signals are cancelledadaptively as this clutter varies. The target detection channel isoperative to detect moving targets and energize suitable targetindicators, and such detection is accomplished in a particularlyeffective manner by virtue of the optimal clutter cancellation affordedby the invention. Knowledge of expected spectra is not necessary, as inknown systems, as the invention provides means for maintaining anacceptable clutter output level as its spectrum varies.

Clutter can be characterized as a kind of nonstationary nonwhite noise,that is, noise which has nonconstant statistics and has a variableenergy per unit bandwidth which is generally of monotonically decreasingspectral density with increasing Doppler frequency. The presentinvention is broadly useful in providing adaptive detection of signalsin the presence of such noise, or other nonstationary noise withdiffering shapes of nonwhite spectra, and is not limited to radar signalprocessing. Nonstationary nonwhite noise is encoun tered, for example,in sonar detection, acoustic or infrared signal detection,electro-optical signal processing and Seismological signal processingsystems, and the invention can be employed in any such system to achieveadaptive noise cancellation to provide a low false alarm rate withminimal sacrifice of signal detection probability.

DESCRIPTION OF THE DRAWINGS The invention will be more fully understoodfrom the following detailed description taken in conjunction with theaccompanyingdrawings, in which:

FIG. 1 is a spectral diagram of typical clutter and target signalsuseful in explaining the operation of the invention;

FIG. 2 is a block diagram of a radar processor embodying the invention;

FIG. 3 is a block diagram of an electronically variable filter employedin the invention;

FIG. 4 is a block diagram of an alternative embodiment of the invention;

FIG. 5 is a diagram of the filter characteristics of the embodiment ofFIG. 4; and

FIG. 6 is a block diagram of a further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION Before proceeding to a discussionof embodiments of the invention, it will be useful to consider thetypical clutter and target spectra illustrated in FIG. 1 in order toexplain the novel signal processing techniques employed in cancellingtime variable clutter signals. The spectral envelopes 5, 6 and 7 shownin FIG. 1 are of a form typically produced by moving trees underdifferent wind conditions. The relatively narrow spectrum 5 is producedby tree movement at low wind velocities, while successively broaderspectra 6 and 7 are produced by tree movements at higher windvelocities. The spectra of two moving targets are shown as envelopes 8and 9. It is seen that the amplitude of the target envelopes aregenerally less than the amplitude of the clutter envelopes and that thetarget envelope 8 is essentially within the spectrum of clutter envelope7. It will be appreciated that targets moving with a particular velocitycan be masked by clutter occuring under certain conditions so that suchtargets, but for the invention, would not be detectable in fixed-filtersystems which utilize high cutoff frequencies to obtain low false alannrate operation under severe clutter conditions. As will be evident fromthe discussion which follows, the present invention provides a novelmeans of discriminating targets from clutter in an optimum manner byadaptively processing clutter and target signals such that time varyingclutter signals are dynamically cancelled.

A block diagram representation of a radar processor embodying theinvention is illustrated in FIG. 2 and is operative to'receive audiosignals derived from the output of a CW radar or the output of asample-and-hold circuit of a pulse radar, these audio signals beingrepresentative of the Doppler returns of moving targets being detected.The Doppler signals are applied to a low pass filter 10, the output ofwhich is applied to an electronically variable high pass filter 12having a first output applied to a feedback control channel 14 and asecond output applied to a target signal detection channel 16. The lowpass filter has a cutoff frequency effective to eliminate target andradar noise components above the Doppler band of interest. Theelectronically variable high pass filter 12 has a eutoff frequency whichis controllably variable between predetermined limits to provide noveladaptive signal processing according to the invention. Preferably, thesecond filter output has a steeper-sloped response than the filterresponse of the first output providing relatively high clutter energyfor operation of the clutter cancellation loop. Both of these outputshave approximately the same instantaneous cutoff frequency which iselectronically variable.

The feedback control channel 14 includes an amplifier 18, a detector21), a clutter residue integrator 22 and a control circuit 241. A signalfrom the first output of filter 12 is amplified and detected inamplifier and detector 18 and 20, respectively, and is then applied toclutter residue integrator 22 which preferably has a time constantseveral times that of matched integrator 30 in the target signal channel16 to prevent the filter from adapting to a target signal, which wouldreduce the probability of detection of that target. Control circuit 24derives from the integrator output signal a control signal which is fedback to electronically variable filter 12 and which is operative toselectively vary the cutoff frequency of filter 12 to optimallydiscriminate between clutter and target signals.

The target signal channel 16 includes an amplifier 26, a detector 28, atime-on-target matched integrator 30, a trigger 32 and an alarm 3 Afteramplification and detection in amplifb er and detector 26 and 28,respectively, the second output signal from filter 12 is integrated bymatched integrator 30 which is optimized for detection of targets withina predetermined velocity range, which corresponds to a given expectedrange of target observation times. Trigger 32 is activated by integratoroutput signals above a predetermined magnitude, the output of thetrigger circuit energizing a suitable alarm 34 which may be for exampleaudible or visual. The alarm can be reset manually once the alarm hasbeen noted and the trigger circuit 32 can be of a latching type whereinthe alarm will continue in operation until it is reset. Theelectronically variable filter 12 may itself have sufficient gain todispense with either or both of amplifiers 18 and 26 in a particularembodiment.

The electronically variable high pass filter 12 can be implemented by avariety of circuits but preferably is an active filter in which precisevariation of its characteristics is accomplished with a control signal.One such filter especially useful in the present invention is describedin detail in copending application Ser. No. 783,290, filed Dec. 12,i968, and assigned to the assignee of the present invention. This filteris illustrated diagrammatically in FIG. 3 and includes an active highpass filter 36 and a controller 38 connected to filter 36 and operativeto selectively vary its cutoff characteristics in an effective andprecise manner. Active filter 36 is essentially a three section RCactive high pass filter having a five pole Butterworth response of 30db. per octave at the output of the third stage and, at the output ofthe second stage, a three pole Butterworth response of 18 db. peroctave. The lower-sloped response output is coupled to control loop 14to provide relatively high clutter energy for control loop operation.Controller 35 is an array of five matched electronically variableresistors which are associated with the three stages of filter 36 suchthat precise variation in the resistive value of the matched array willadjust the filter cutoff frequency in linear inverse relation to theresistance values without affecting the basic high pass response. Thevariable resistance elements employ combinations of photosensitiveresistors and light sources wherein variation of light intensity causedby variation in the energizing electrical signal results in acorresponding variation in resistive value. The control signal for thispreferred type of filter is a temperature stable variable currentderived from a current source driver embodied in control circuit 24, thevariable current controllably energizing the light sources of controller38 which control the photosensitive resistors of controller 38 which, inturn, adjusts the response of active filter 36.

The input to active filter 36 is the low pass filtered audio signalsfrom the radar, while the input to controller 38 is the control signalfrom control circuit 24 in control loop 14 which provides the intendedfeedback control. The active filter can have an overall gain throughoutthe three sections of 30 to 50 db. or more to provide high dynamicrange, and in this instance, only one external amplifier 18 need byemployed in the illustrated embodiment of FIG. 2 to compensate the gainof control loop 14 with that of the overall filter since the secondstage filter output does not experience the same gain as the third stageoutput.

The active filter 12 is operative in response to the frequency and/oramplitude of input signals applied thereto to vary its cutoff frequencyaccordingly to optimally cancel clutter signals and thereby permitoptimum target detection. The filter response is automatically adjustedsuch that the signalto-clutter residue ratio is maintained as high aspossible under particular clutter conditions. For example, if aparticular clutter spectrum is separated in frequency from the spectrumof a moving target, the filter response is automatically adjusted tocancel only those frequencies in the clutter region, thereby providingrelatively wideband target detection. On the other hand, if the clutterspectrum spreads in such a way as to be near or partially mask thetarget spectrum, the filter response adapts to the new condition tocancel clutter, lowering the signal-to-clutter residue ratio but yetproviding optimal target detection under the circumstances. Undercertain conditions where the clutter spectral spread is very rapidlyincreasing, there may be a transient filter adaptation period duringwhich time the probability of false alarm increases, but such conditionsare not usual in most operating environments and in the event that itdoes occur only infrequent false alarms may result.

The target detection channel can be implemented in various waysdepending upon the degree of sophistication required in a particularoperating environment. For example, FIG. 4 illustrates target detectionapparatus operative to provide optimal detection over a wider range ofexpected target velocities than illustrated in FIG. 2 by providing aplurality of channels matched to particular bands of velocity. Referringto FIG. 4, a multichannel processor is shown for use in place of channel16 of H6. 2. Three channels are illustrated, one for low velocitytargets, one for medium and one for high velocity targets. Each channelcontains a predetection filter 40a, 40b and 400, a detector 42a, 42b and420, and a time-on-target matched integrator 44a, 44b and 440, eachmatched integrator having a response providing target matching for thosetargets whose velocities correspond to the Doppler frequencies passed bythe associated preceding predetection filters. The outputs of theseintegrators are combined in OR circuit 46, the output of which isapplied to a trigger circuit 48 which, in turn, drives an alarm 50.Predetection filters 40a, 40b and 400 have effective pass bands toprocess targets .of respective velocity ranges; however, filters 40a and400 need not be band pass filters as such. Filters 40a, 40b and We aredriven by signals which have been low pass filtered by input filter 10,and high pass filtered by active filter 12. The low pass response offilter 40a combined with the high pass response of filter 12 thereforeprovides the intended band pass response for low velocity targets. Inlike manner, the high pass response of filter 40c combined with the lowpass response of filter it) provides the band pass response desired forhigh velocity targets. Filter 40b is a band pass filter for intermediatevelocity target detection.

The filter response characteristicsof the embodiment of FIG. 4 aredepicted in FIG. 5. Dotted curves 52 and 54 show the respective minimumand maximum cutoff frequency of variable filter 12. Solid curves 56, 58and 60 show the low pass response of filter 40a, the band pass responseof filter 40b and the high pass response of filter 40c, respectively.The low pass response of input filter is shown as broken curve 62. It isevident that the combined response of filters l2 and 40a provides apredetermined pass band, as do filters l0 and 400.

FIG. 6 illustrates another embodiment of the invention wherein novelclutter cancellation is employed in a balanced target detectionprocessor. In this embodiment, quadrature components of audio signalsderived from a coherent radar are applied to a respective one of a pairof adaptive filters 64 and 66, each having a respective clutter residuefeedback control loop 68 and 70, as described hereinabove. The secondoutput of the adaptive filters are applied to single-sideband audioconversion circuitry 72'which provides a pair of outputs, one fortargets moving away from the radar and the other fortargets movingtoward the radar. Each output is then applied to a respective filterbank 74 and 76, each of which is similar to the multichannel filters ofFIG. 4. The output'of each filter bank is applied to a respectivedetector 78 and 80, the outputs of which are combined in summing circuit82. The filter banks are as in the embodiment of FIG. 4 wherein eachfilter accommodates a range of target velocities. In this embodiment,however, a single integrator is employed for each velocity channel andeach integrator is driven by the summed output of a positive andnegative polarity detector which has been driven by the outputs of theincoming and outgoing channels respectively. The outputs of theintegrators 84 are combined in an OR circuit 86 and applied to a triggercircuit 88 which is operative to energize the alann 90 when a thresholdhas been. crossed. The integrator can operate on either positive ornegative signals; thus, the trigger circuit can activate an alarm oneither positive or negative signals which have exceeded thepredetermined threshold level.

Although the invention has been described in conjunction with cluttercancellation in a radar processor, it will be appreciated that theinvention is, as described hereinabove, also useful in the processing ofsignals generally wherein nonstationary nonwhite noise is to bediscriminated against. Such nonstationary nonwhite noise occurs, forexample, in electrooptical, acoustic, infrared, seismological and sonarsystems. In addition, systems embodying the invention can be implementedby well-known analog, digital or hybrid circuitry as appropriate inparticular instances. Accordingly, the invention is not to be limited bywhat has been particularly shown and described, as various modificationsand alternative implementations will occur to those versed in the artwithout departing from the spirit and true scope of the invention.

Iclaim:

l. A system for detecting moving targets in the presence of clutter,comprising:

input filter means for receiving an input signal which includes a targetsignal in the presence of clutter;

an electronically variable filter having first and second outputs;

means coupled to said first output and operative in response to saidclutter to provide a control signal representative of the shape andenergy content of the input clutter envelope;

feedback means for applying said control signal to said electronicallyvariable filter to selectively and adaptively vary the frequencyresponse thereof thereby to dynamically maintain a predeterminedsignal-to-clutter ratio even in the presence of variable clutterspectra; and

target detection means coupled to said second output and operative todetect moving targets of a selected velocity range substantially free ofsaid clutter.

2. A system according to claim 1 further including alarm means forindicating the detection of a moving target.

3. A system according to claim 1 wherein said electronically variablefilter is a high pass filter, the cutoff frequency of which is variablein response to said control signal.

4. A system according to claim 3 wherein the first output of saidelectronically variable filter has a lower-sloped response than theresponse of said second output.

5. A system according to claim 3 wherein the means cou pled to saidfirst output includes a detector and a clutter residue integrator, andsaid control signal is derived from the output signal of saidintegrator.

6. A system according to claim 5 wherein said target detection meansincludes a detector, a matched integrator, a target indicator andthreshold circuitry operative to activate said target indicator.

7. A system according to claim 6 wherein said clutter residue integratorhas a time constant greater than that of the matched integrator of saidtarget detection means.

' 8. A system according to claim 5 wherein said target detection meansincludes a plurality of predetection filters each' effective to passselected different ranges of Doppler frequencies, a plurality ofdetectors, each associated with a respective predetection filter, aplurality of matched integrators each associated with a respectivedetector and each having a response corresponding to said differentranges of Doppler frequencies, and threshold circuitry operative inresponse to a signal from any of said matched integrators to indicate adetected moving target.

9. A system for detecting signals in the presence of nonstationarynonwhite noise, comprising:

means for receiving an input signal which includes a desired signal inthe presence of nonstationary nonwhite noise;

an electronically variable filter operative to receive said input signaland noise and having a predetermined selectively variable frequencyresponse;

means coupled to an output of said filter for deriving a control signaltherefrom in response to and representative of the shape and energycontent of the envelope of said input noise;

means for applying said control signal to said electronically variablefilter to selectively and adaptively vary the response thereof therebyto dynamically maintain a predetermined signalto-noise ratio even in thepresence of variable noise spectra; and

means coupled to an output of said filter for detecting said desiredsignal.

10. A system according to claim 9 wherein said electronically variablefilter includes first and second outputs, the first output being coupledto said means for deriving a control signal, the second output beingcoupled to said signal detectmg means.

II. A system for detecting moving targets in the presence of cluttercomprising:

means for receiving from a coherent radar, quadrature signals whichinclude a target signal in the presence of clutter;

first and second electronically variable filters each operative toreceive a respective quadrature signal and each having first and secondoutputs;

a respective clutter control loop coupled to the first output of each ofsaid filters, each control loop including;

means operative in response to said clutter to provide a control signalrepresentative of the spectral characteristics of said clutter; andmeans for applying said control signal to said filter to selectively andadaptively vary the response thereof thereby to dynamically maintain apredetermined signal-to-clutter ratio even in the presence of variableclutter spectra;

single-sideband means coupled to the second output of each of saidfilters and operative to produce first and second signals respectivelyrepresentative of targets moving toward and away from the radar; and

means for detecting the presence of moving targets of selected velocityranges.

12. A system for detecfing signals in the presence of nonstationarynonwhite noise, comprising:

' means for receiving an input signal which includes a desired r signalin the presence of nonstationary nonwhite noise; an electronicallyvariable high pass filter having first and second outputs and operativeto receive said input signal and noise, said first output having alower-sloped response thansaid second output; means coupled to saidfirst output and operative in response UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 3 ,576 564 Dated April 27 1971Inventofld) Aaron A. Galvin It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In the references cited, Patent 3,776,426 (Altman) ShOL read 2,776,426.

Column 6, line 62, delete after "including".

Signed and sealed this 28th day of March 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

1. A system for detecting moving targets in the presence of clutter,comprising: input filter means for receiving an input signal whichincludes a target signal in the presence of clutter; an electronicallyvariable filter having first and second outputs; means coupled to saidfirst output and operative in response to said clutter to provide acontrol signal representative of the shape and energy content of theinput clutter envelope; feedback means for applying said control signalto said electronically variable filter to selectively and adaptivelyvary the frequency response thereof thereby to dynamically maintain apredetermined signal-to-clutter ratio even in the presence of variableclutter spectra; and target detection means coupled to said secondoutput and operative to detect moving targets of a selected velocityrange substantially free of said clutter.
 2. A system according to claim1 further including alarm means for indicating the detection of a movingtarget.
 3. A system according to claim 1 wherein said electronicallyvariable filter is a high pass filter, the cutoff frequency of which isvariable in response to said control signal.
 4. A system according toclaim 3 wherein the first output of said electronically variable filterhas a lower-sloped response than the response of said second output. 5.A system according to claim 3 wherein the means coupled to said firstoutput includes a detector and a clutter residue integrator, and saidcontrol signal is derived from the output signal of said integrator. 6.A system according to claim 5 wherein said target detection meansincludes a detector, a matched integrator, a target indicator andthreshold circuitry operative to activate said target indicator.
 7. Asystem according to claim 6 wherein said clutter residue integrator hasa time constant greater than that of the matched integrator of saidtarget detection means.
 8. A system according to claim 5 wherein saidtarget detection means includes a plurality of predetection filters eacheffective to pass selected different ranges of Doppler frequencies, aplurality of detectors, each associated with a respective predetectionfilter, a plurality of matched integrators each associated with arespective detector and each having a response corresponding to saiddifferent ranges of Doppler frequencies, and threshold circuitryoperative in response to a signal from any of said matched integratorsto indicate a detected moving target.
 9. A system for detecting signalsin the presence oF nonstationary nonwhite noise, comprising: means forreceiving an input signal which includes a desired signal in thepresence of nonstationary nonwhite noise; an electronically variablefilter operative to receive said input signal and noise and having apredetermined selectively variable frequency response; means coupled toan output of said filter for deriving a control signal therefrom inresponse to and representative of the shape and energy content of theenvelope of said input noise; means for applying said control signal tosaid electronically variable filter to selectively and adaptively varythe response thereof thereby to dynamically maintain a predeterminedsignal-to-noise ratio even in the presence of variable noise spectra;and means coupled to an output of said filter for detecting said desiredsignal.
 10. A system according to claim 9 wherein said electronicallyvariable filter includes first and second outputs, the first outputbeing coupled to said means for deriving a control signal, the secondoutput being coupled to said signal detecting means.
 11. A system fordetecting moving targets in the presence of clutter comprising: meansfor receiving from a coherent radar, quadrature signals which include atarget signal in the presence of clutter; first and secondelectronically variable filters each operative to receive a respectivequadrature signal and each having first and second outputs; a respectiveclutter control loop coupled to the first output of each of saidfilters, each control loop including; means operative in response tosaid clutter to provide a control signal representative of the spectralcharacteristics of said clutter; and means for applying said controlsignal to said filter to selectively and adaptively vary the responsethereof thereby to dynamically maintain a predeterminedsignal-to-clutter ratio even in the presence of variable clutterspectra; single-sideband means coupled to the second output of each ofsaid filters and operative to produce first and second signalsrespectively representative of targets moving toward and away from theradar; and means for detecting the presence of moving targets ofselected velocity ranges.
 12. A system for detecting signals in thepresence of nonstationary nonwhite noise, comprising: means forreceiving an input signal which includes a desired signal in thepresence of nonstationary nonwhite noise; an electronically variablehigh pass filter having first and second outputs and operative toreceive said input signal and noise, said first output having alower-sloped response than said second output; means coupled to saidfirst output and operative in response to said noise to provide acontrol signal representative of input noise characteristics; feedbackmeans for applying said control signal to said filter to selectively andadaptively vary the response thereof thereby to maximize the ratio ofsignal to noise; and means coupled to said second output for detectingsaid desired signal.